This invention relates to air-source heat pumps. More particularly, this invention relates to a new and improved air source heat pump especially suitable for use in normally colder climates.
The air-source heat pump system is the most prevalent type of heat pump used in the world today. This is the case whether one is discussing room units, residential central type, ductless splits, or rooftop commercial systems.
Although the air-source concept in general has a high application potential worldwide, its popularity in the United States and elsewhere has been greatest in mild climate areas. This is because the compressor-derived heating capacity of conventional units declines rapidly as the outdoor ambient falls, due, in the most part, to the large increase in specific volume (i.e., decrease in density) of the outdoor coil generated refrigerant vapor as ambient (outdoor) temperature falls (see FIG. 2). This fall in compressor-derived heating capacity is obviously opposite to the heating requirement, which generally increases in proportion to the fall in outdoor ambient temperature. This problem is illustrated in FIG. 1, which shows a plot of heating requirements vs. outdoor ambient temperature and the heating performance of a 3 Ton Lennox HP 22-411 fixed speed scroll compressor heat pump system. As shown in FIG. 1, the heating requirements (line 2) increase as ambient temperature decreases, going from zero (0) BTU/hr at 65.degree. F. to 72,000 BTU/hr at 5.degree. F. outdoor ambient temperature. The compressor derived heating capacity is shown at line 4.
When a typical prior art heat pump operates below its balance point, (about 40.degree. F. in FIG. 1), supplemental heating is required. The most prevalent form of supplemental heat used is electric resistance. In other than mild climates, this use of supplemental electric resistance heat puts the air-source heat pump at an economic disadvantage to a consumer as compared with other forms of heating, because of the high cost of electric resistance heating. Electric utilities are also concerned because of the associated high peak power demand during cold weather.
One of the areas for improvement of air source heat pump systems lies in the efficient recovery of the low grade heat energy remaining in the condensed refrigerant liquid leaving the system condenser. If this remaining energy is recovered and then returned to the heating side of the system, rather than being further thermally degraded and sent to the system evaporator (as is now the case), very significant increases in overall compressor-derived heating capacity can be made.
The basic problem here is that after the refrigerant has been fully liquified in the condenser, there is still a large amount of energy left in the warm liquid. This remaining energy serves to evaporate a large portion of the liquid itself during the normal pressure reduction process that occurs across the system expansion device. Depending on the refrigerant utilized, and the temperatures existing between the evaporator and the condenser, as much as one-half, or even more, of this liquid can be evaporated during the normal pressure reduction process across the system expansion device. Obviously if liquid has already evaporated, it cannot be again evaporated in the system evaporator, and thus cannot absorb energy from the outside air. However, the net resulting vapor must pass through the system evaporator anyway, creating additional pressure drop along its way, and then must be fully compressed to the condensing level by the compressor. If the compressor must induct this useless vapor, it can only induct a smaller amount of useful vapor. However, compressor power must be expended to compress the total amount of vapor that has been inducted into the compressor. This is not a reasonable process for air-source heat pumps operating in other than the milder ambient temperatures.
Referring again to FIG. 1, the required heat input to an occupied space (line 2) increases in direct proportion to the fall in outdoor ambient temperature whereas the compressor derived heating capacity (line 4) declines rapidly. This is because the heat output of any heat pump is essentially proportional to the weight flow of refrigerant vapor entering the system condenser. FIG. 2 shows what happens to the specific volume of evaporator generated refrigerant vapor as the evaporating temperature falls. At a 50.degree. F. outdoor ambient supporting a 40.degree. F. evaporating temperature, the specific volume is about 0.46 cubic feet per pound of generated vapor whereas at 0.degree. F. outdoor ambient with an evaporating temperature of -25.degree. F., the specific volume is 1.6 cubic feet per pound of generated vapor. This is 3.5 times the volume of vapor per pound compared to the 50.degree. F. ambient level. Further to this, more than 4 times the amount of heat is required at 0.degree. F. as is required at 50.degree. F. This means that a dramatic increase of refrigerant vapor is required at 0.degree. F. ambient as compared to 50.degree. F. ambient in order to adequately match the heat energy requirement.
In addition, if the entire space heating requirement at 0.degree. F. outdoor ambient is to be supplied by compressor derived heating capacity, the air flow across the heating coil of the condenser must be such that the indoor delivered air temperature will be at least 110.degree. F. in order to provide adequate freedom from a sensation of cool drafts. This in turn will cause the system condensing temperature to rise to about 140.degree. F. considering a reasonably sized indoor coil surface. The end result of all this is to cause overall system operating compression ratios to rise to the point where it becomes unrealistic to even consider the use of present day technology for such an application.
The various factors presented above clearly show that present day air source heat pumps do not even come close to doing the job that is required for efficient heating in cold climates.